So far, consensus on the migratory behaviour of marine mammals has evaded the scientific community. Large whale migrations are annual, round-trip movements from high latitudes in the summer, where they capitalise on the seasonal abundance of prey, to lower latitudes in winter. Baleen whales – the group that filter plankton from the water – are known to make the longest migrations of any mammal: gray whales complete arduous annual round trips of 15,000–25,000 km from the Bering Sea to Mexico. However, why whales travel to the tropics has remained a mystery. It’s been known for over a century that whales calve when they go to the tropics, but the question that has not been answered is ‘why’. Some researchers think that migrations are for feeding; others suggest that warmer waters improve the chances of conception; a few even posit the search for shallower waters to protect calves from predators. Nonetheless, understanding the causes for whale migrations is key to knowing how resources are dispersed throughout our oceans.
Dr. Robert Pitman and his team at the Marine Mammal Institute of Oregon State University, tracked the movement of 62 killer whales in coastal waters of Antarctica from 2009 to 2016. Each sub-type of killer whale undertook long distance, high speed migrations to lower latitudes: one individual journeyed over 11,000 km from 78°S to 30°S – the longest killer whale migration ever recorded. Another, tagged off the Western Antarctic Peninsula, migrated to the waters of southern Brazil and back, twice, after spending just 71 days in the chill of the Antarctic waters. Remarkably, this is the first record of any animal completing the same, long-distance, round-trip migration twice in the same year.
‘They went off to the east coast of South America to the edge of the tropics, turned around and came straight back – they didn’t spend any time feeding anywhere, and they didn’t leave [Antarctica] because they ran out of food,’ says Pitman.
Instead, the team think that the whales journeyed to the warmer waters to regulate the natural cycling of their skin. In extreme cold, whales are able to restrict blood flow to their skin surface, conserving heat. This economy comes at a metabolic cost – skin cells are less able to renew and maintain their sleek surface, accumulating striking yellow ‘diatoms’. When the whales return to warmer climates, they can properly remove the diatoms and regulate their skin physiology. ‘Skin molting is a self-cleaning operation, allowing them to mend injuries, remove bacteria, and even grow. Some individuals that we recurrently photographed had really bad yellowing in Antarctic waters. They would return looking as if they just had been to the car wash,’ says Pitman.
The migratory return to warmer waters is a known trait of beluga whales, bowhead whales, and possibly narwhals. As such, Pitman’s team suspect that the behaviour may be a rule for high-latitude cetaceans.
Pitman is passionate about the value of understanding whale migrations: ‘prior to commercial whaling, literally millions of tonnes of animal flesh went from high to low latitudes, moving around ocean resources on a huge scale, and contributing to local ecosystems.’ Whales are engineers of the ocean biome; killer whales consume huge amounts of prey, while baleen whales regulate the density of plankton. Additionally, whales store great volumes of carbon through their diet, transporting it to deep sea ecosystems when they die. Pitman’s research is the latest research pioneering our understanding of long-distance migrations – a movement that will allow us to better understand how ecosystem resources, such as carbon and iron, are distributed throughout our oceans.